The present invention relates to a gear pump which is particularly suitable for the delivery of liquids which give rise to deposits within the pump, in particular on the faces of the gearwheels.
The delivery of silver halide emulsions for photographic materials, and in particular their accurate metering, by means of gear pumps is known from EP-A 0 625 641. It is also known that in the course of the operation metallic silver is deposited, in particular between the faces of the gearwheels and the front and rear covering plate, as quite a narrow gap is provided here in order that the gearwheels can rotate within the housing of the gear pump, which is formed inter alia by the front and rear covering plates (see exploded drawing in EP-A 0 625 641).
According to prior art, the deposition of silver is caused by electrolysis of the silver halide emulsion and can be avoided if the parts of the pump which come into contact with the silver halide emulsion are composed of cobalt alloys of the type referred to as "stellite".
Long-term tests have shown that an effective prevention of the deposition of silver is not successfully achieved in this way.
When the deposited layer of silver has attained a certain thickness, determined by the gap width between the faces of the gearwheels and the covering plates, the driving motor is no longer capable of rotating the gearwheels and comes to a halt. Apart from mechanical damage, there is also a halt to the production.